The present invention is generally directed to continuous glass fibers for use in high temperature applications.
The use of continuous glass fibers in high temperature environments for acoustical and thermal insulating applications can be in the form of wound strands, packed fiber, or texturized strands. Texturization is accomplished by injecting the strands into cavities through a nozzle with compressed air. This process fluffs-up the strands and creates a fiber pack that is much lighter in density. However, applications that require these insulating characteristics at temperatures greater than 850xc2x0 C. are limited in terms of the glass composition that can withstand the high temperature environment. An example of a texturized continuous glass fiber product is the use of E-glass and Advantex(copyright) glass fiber in mufflers. Texturization produces a fluffy fiber pack that is a better insulator, both thermally and acoustically. Advantex is a registered trademark of Owens Corning for boron-free glass fibers.
Filling mufflers with one or more lengths of fiberglass wool is disclosed by U.S. Pat. No. 4,569,471 to ingemansson. The fiberglass wool is inserted into a space in a container by feeding a multifilament fiberglass thread into one end of a nozzle and advancing the thread through the nozzle with the aid of compressed air which is blown into the nozzle to cause the fibers of the thread to separate and become entangled, so that the thread emerges from the other end of the nozzle as a continuous length of fiberglass wool, which is blown by the effect of the compressed air through an opening into the container space at the same time as air is evacuated from the space.
The standard glass composition for making continuous glass fiber strands isxe2x80x9cExe2x80x9d glass. E glass, is the most common glass for making textile and reinforcement glass fibers. One advantage of E glass is that its liquidus temperature is approximately 200xc2x0 F. (93xc2x0 C.) below its forming temperature, the temperature at which the viscosity of the glass is customarily near 1000 poise. E glass melts and refines at relatively low temperatures and has a workable viscosity over a wide range of relatively low temperatures, a low liquidus temperature range, and a low devitrification rate. E glass compositions allow operating temperatures for producing glass fibers around 1900xc2x0 F. to 2400xc2x0 F. (1038xc2x0 C. to 1316xc2x0 C.) where the liquidus temperature is approximately 2100xc2x0 F. (1149xc2x0 C.) or lower. The ASTM classification for E-glass fiber yarns used in printed circuit boards and aerospace applications defines the composition to be 52 to 56 weight % SiO2; 16 to 25 weight % CaO; 12 to 16 weight % Al2O3; 5 to 10 weight % B2O3; 0 to 5 weight % MgO; 0 to 2 weight % Na2O and K2O; 0 to 0.8 weight % TiO2; 0.05 to 0.4 weight % Fe2O3; 0 to 1.0 weight % Fluoride.
However, E-glass fiber containing 5 to 10 weight percent B2O3 is limited to temperatures less than 680-690xc2x0 C. since it will xe2x80x9csinterxe2x80x9d at higher temperatures. Sintering is defined as the coalescence of filaments at contact points through viscous flow. Viscous flow typically occurs at temperatures greater than the annealing point. The annealing point of a glass is defined as the temperature corresponding to a viscosity of 1013 Poise. When filaments coalesce, the insulating ability of the fiber pack is reduced. In addition, a texturized fiber pack becomes brittle after sintering and can break into fiber fragments when stressed.
Boron free E-Glass fibers sold under the trademark Advantex(copyright) and disclosed in U.S. Pat. No. 5,789,329 offer a significant improvement in operating temperature over boron containing E-glass. Advantex(copyright) glass fiber fits the ASTM definition for E-glass fiber used in general use applications which is 52 to 62 weight % SiO2; 16 to 25 weight %CaO; 12 to 16 weight % Al2O3; 0 to 10 weight % B2O3; 0 to 5 weight % MgO; 0 to 2 weight % Na2O and K2O; 0 to 1.5 weight % TiO2; 0.05 to 0.8 weight % Fe2O3; 0 to 1.0 weight % Fluoride. However, Advantex(copyright) glass fiber begins to sinter at temperatures greater than 740-750xc2x0 C.
Other than fused silica, S-glass is the only commercially available continuous glass fiber that can operate at temperatures greater than 850xc2x0 C. S-Glass is a family of glasses composed primarily of the oxides of magnesium, aluminum, and silicon with a certified chemical composition which conforms to an applicable material specification and which produces high mechanical strength. S-Glass has a composition of approximately 65 weight % SiO2; 25 weight % Al2O3; 10 weight % MgO. S-glass has a glass composition that was originally designed to be used in high strength applications such as ballistic armor. Therefore, even though S-glass can perform at high temperatures (up to 900xc2x0 C. for short periods of time), it is not the optimal composition for a high temperature insulating glass fiber.
It has been determined that failure of the texturized fiber pack occurs at temperatures exceeding the annealing point (1013 Poise). A close approximation to the annealing point is the glass transition temperature, or Tg. Since the annealing point of most of the glasses presented in this invention are greater than what can be measured by most commercially available tests, Tg was used as a means of determining the upper use temperature of the fiber.
Both E-glass and Advantex(copyright) experience significant sintering at temperatures greater than the annealing point. S-glass, however, resists sintering at temperatures above the annealing point due to phase separation. S-glass fibers are formed by cooling very rapidly from the molten state into a solid, homogeneous glass. The rapid cooling during fiber forming does not allow the glass sufficient time to phase separate during the cooling period. Upon reheating, S-glass will phase separate from a homogeneous glass into 2 separate glasses with different compositions in the temperature range between the glass transition temperature (Tg and the miscibility limit. The glass transition temperature is approximately 820xc2x0 C. for S-glass, whereas the miscibility limit is not known. The phase separation of S-glass is a slow process that results in a 2 phase glass including a continuous SiO2 rich phase that has a greater viscosity than the original homogeneous glass and an SiO2 poor phase which has a lower viscosity. The overall viscosity of the fiber is determined by the morphology and composition of the SiO2 rich, high viscosity phase. The higher effective viscosity of the phase-separated glass allows the fiber to operate at greater temperatures than a homogeneous fiber. Phase separation in S-glass is a slow process and some viscous flow occurs prior to the development of the continuous high viscosity phase, which can result in reduced albeit acceptable performance.
The invention, in part, is a glass composition suitable for the formation of continuous glass fiber that is suitable for use in high temperature applications. The composition of the present invention may be inexpensively formed into glass fiber using low cost direct melting technology because of a relatively low forming viscosity and once formed into fibers resists softening and annealing because of a relatively high glass transition temperature. The composition of the present invention is more appropriately expressed in terms of mole percent rather than weight percent due to the dramatically different atomic weights of the alkaline earth oxides. The composition of the present invention is 60-72 mole percent SiO2, 10-20 mole percent Al2O3, 14.0 to 22.0 mole percent RO where RO equals the sum of MgO, CaO, SrO and BaO, 0 to 5 mole percent ZrO2, and 0 to 3 mole percent alkali. In a preferred embodiment the glass composition is substantially 61-68 mole percent SiO2, 15-19 mole percent Al2O3, 15-20 mole percent alkaline earth oxide, 0 to 3 mole percent ZrO2, and 0 to 3 mole percent alkali metal oxide. The composition may also contain not more than about 4 mole percent of at least one oxide or halogen selected from the group consisting of ZnO, SO2, F2, B2O3, TiO2 and Fe2O3.
The desired properties of the present invention are: a viscosity of 1000 poise at a forming temperature of from 2500xc2x0 F. (1371xc2x0 C.) to 2750xc2x0 F. (1510xc2x0 C.), a liquidus temperature at least 50xc2x0 F. (28xc2x0 C.) below the forming temperature, and a glass transition temperature greater than 1517xc2x0 F. (825xc2x0 C.). The glass transition point (Tg) is a measure of a low temperature viscosity and the forming viscosity is a high temperature viscosity. One would expect these viscosities to be related. However, the glasses of the present invention have reduced forming viscosities and increased glass transition temperatures as compared with S-glass. This greater temperature dependence of the glass viscosity allows for inexpensive forming of fibers which exhibit good high temperature characteristics such as increased resistance to fiber-to-fiber coalescence and slumping of a fiber pack formed of the inventive compositions.